278 RECOED OF CURRENT RESEARCHES RELATING TO 



opening and closing of the pores in the adult is due to the same 

 property. 



The apertures in the ectoderm are of two kinds : permanent pores 

 with rounded apertures, ^—^ mm. in diameter, communicating with 

 the canals which lead into the gastric cavity (dermal ostia) ; and small 

 temporary pores, invisible to the naked eye, and leading into the sub- 

 dermal spaces (dermal pores). The subdermal spaces are lined by a 

 pavement epithelium, but whether or not this is ectodermal is not 

 certain. 



Mesoderm. — This layer consists of a gelatinous matrix contain- 

 ing amoeboid corpuscles, which are often spindle-shaped, and in the 

 neighbourhood of the pores take on a sphincter-like arrangement and 

 probably act as muscles. 



As in Spongilla, Chondrosia, &c., cells containing amyloid substances 

 occur, though infrequently, in the mesoderm. Probably this rarity 

 was due to the fact that, in the breeding season, when the examination 

 was made, the reserve material was largely used up. 



Endoderm. — The author considers that the endoderm lines both 

 the gastric cavity close up to the edge of the osculum, and the whole 

 system of afferent canals. There are two kinds of endoderm cells : 

 flagellate, lining the ciliated chambers, and pavement lining the 

 canals. The latter are made out by treatment with silver nitrate. 



The Canal System. — This includes a central portion consisting 

 of the large gastric cavity, and a peripheral portion consisting of 

 radial canals opening by gastric ostia into the central portion. These 

 canals at their outer ends open directly on the surface, but amongst 

 them are irregularly disposed canals of various sizes, which open into 

 the subdermal cavities ; these latter communicating with the exterior 

 by the dermal pores. 



Development. — The sexes are distinct, and are outwardly recog- 

 nizable by differences of colour, which also differs in the female 

 according to whether it contains unfertilized eggs, dividing eggs, or 

 larvae. 



The spermatozoa consist of an elongated somewhat curved head, 

 and a long tail. They are formed from mesoderm cells, and are 

 enclosed in special capsules lined with epithelium. 



The eggs are also specially modified mesoderm cells, exhibiting 

 while young amoeboid movements. The ripe eggs are enclosed in a 

 definite capsule, and are rendered very opaque by the number of yolk- 

 granules contained in them. 



Yolk-division is total, but unequal, the seven-celled stage consist- 

 ing of six micromeres and one macromere. By further division a 

 number of macromeres are formed at the future posterior end of the 

 animal, and represent the endoderm, while the micromeres, undergoing 

 more rapid division and growing over the macromeres, form the 

 ectoderm. This phase represents, according to Keller, the gastrula 

 stage of development, the blastopore being represented by the yolk- 

 plug (Dotterpfropf) formed by the endoderm cells. The gastrula is 

 thus formed by epiboly. 



The ectoderm cells become columnar and acquire cilia, the embryo 



